Objectives: The aim of the study was to investigate the effectiveness of erythromycin in preventing intrauterine infection caused by group B streptococcus (GBS). Material and methods: The study included 20 pregnant women with GBS-positive screening or whose laboratory screening was not available, who delivered between April 17, 2013 and July 22, 2013. The women were given 600 mg of erythromycin intravenously. After delivery, blood was drawn in parallel from maternal antecubital vein and umbilical cord artery. Serum erythromycin concentrations were evaluated using enzyme-linked immunosorbent assay (ELISA) kit. Statistical analysis for measurable and non-measurable characteristics were performed, correlation coefficients for each pair of variables were calculated in order to investigate the sought dependence. Results: Mean placental transfer of erythromycin was 2.04%. There was a high correlation between umbilical artery serum and maternal serum erythromycin concentration. Selected variables of mothers in the control group had no effect on serum erythromycin concentration in the umbilical artery. Conclusions: Transplacental transfer of erythromycin is limited (2.04%). Intravenous application of erythromycin at a dose of 600 mg does not allow to achieve the value of MIC50 and MIC90 for erythromycin against strains S. agalactiae in umbilical artery serum, what suggests a compromised efficacy in the treatment of intrauterine fetal infections. At the same time, the placenta seems to be an effective barrier reducing fetal exposure when this macrolide is used to treat maternal infections.